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American scientists have developed a more efficient fuel cell which runs on everyday energy sources such as methane, raising the prospect of new types of portable generators and perhaps cleaner, greener cars.

The 'Penn fuel cell' developed by Raymond J Gorte and colleagues at the University of Pennsylvania and reported in the latest issue of Nature is a type of battery that draws electric current from chemical reactions. The cell provides power by oxidising methane or other hydrocarbons, such as petrol, electrochemically - a cleaner and more efficient process than just burning the gas.

Currently, methane needs to be converted to hydrogen before it can be used in fuel cells. Gorte's team eliminates this wasteful process. Their fossil-fuel cell churns out about one-tenth of the power of a hydrogen fuel cell, but the group is confident that this will improve.

According to Gorte, "hydrogen had proven too costly and dangerous for widespread use. While earlier studies have successfully generated electricity from methane, the Penn cell is more versatile and effective than predecessors."

Advocates of 'green energy' say fuel cell technology is so efficient it dramatically reduces the amount of fuel needed and the amount of waste produced per kilowatt of electricity generated. The Penn cell produces only water and carbon dioxide.

The Penn fuel cell works like a battery to combine oxygen and hydrocarbon molecules to produce enough free electrons to generate electricity. Unlike a battery, however, it does not run down or need recharging as long as it is supplied with fuel.

The Pennsylvania researchers claim also to have got around a major technical barrier to fuel cell operation - the buildup of carbon in the cells from hydrocarbon combustion.

Fuel cell technology is also actively being developed in Australia by companies such as Ceramic Fuel Cells Ltd, who have also been developing methane as a fuel source.

Dr Karl Foger who is general manager of technology development at Ceramic Fuel Cells Ltd said the US research was at a very early stage. He said the research was based only on a single cell, and that "real life" operating conditions with a multiple cell system would be very different.

"Although the researchers seem to have shown that carbon deposition (a major problem with fuel cell operation) does not occur in the fuel cell, they did see tar-like residues in the feed tube.

"A real fuel cell system incorporates fuel pre-heaters, feed pipes and manifolds, and it would be extremely difficult to prevent decomposition of C2+ hydrocarbon fuels and carbon deposition in these system components."

Dr Foger said Ceramic Fuel Cells Ltd was looking at a "total systems approach" where methane and steam together were fed into cells to prevent carbon deposition.